Motors, based on the 2020 NEC - Part 1

Figure 01

Note: This article is based on the 2020 NEC.

Do you know the basics of Article 430, including what overload protection is and what the requirements are for overload protection?

Article 430 contains the specific rules for conductor sizing, overcurrent protection, control circuit conductors, controllers, and disconnects for electric motors. The installation requirements for air-conditioning and refrigeration equipment are in Article 440; those supplement or amend Article 430 requirements.

Article 430 is long and complex, because motors are complex. They are inductive loads with a high-current demand at start-up (typically six or more times the running current). This makes overcurrent protection for motor applications necessarily different from the overcurrent protection for other types of equipment. So, do not confuse general overcurrent protection with motor protection”you must calculate and apply them differently using the rules in Article 430. Figure 01

Table FLC versus motor nameplate current rating
The size of conductors supplying equipment covered by Article 430 must be selected from the ampacity tables per 310.15 or be calculated per 310.14(B) [430.6].

Determine motor current ratings using:

• 430.6(A)(1) Table Full-Load Current (FLC). Use the motor full-load current ratings in Tables 430.247 through 250 to determine conductor sizing [430.22] and the branch-circuit short-circuit and ground-fault overcurrent protection size [430.52 and 430.62].
• 430.6(A)(2) Motor Nameplate Current Rating (FLA). Overload devices and conductor sizing for other than continuous-duty motors must be sized based on the motor nameplate full-load ampere (FLA) rating per 430.31.
  

Motor locations
Locate motors so they have adequate ventilation and can readily be maintained. [430.14(A). Locate open motors with commutators or collection rings so they don't spew sparks onto combustible material [430.14(B)].

Highest rated motor
You determine the highest rated motor of a group when you need to calculate conductor size and the short-circuit ground-fault protective device and the conductors are supplying more than one motor [430.24]. The highest rated motor of the group is the one with the largest full-load current (FLC) rating as listed in Tables 430.247 through 250 [430.17].

Motor conductor size
Branch-circuit conductors to a single motor in a continuous duty application must have an ampacity of at least 125 percent of the motor's full-load current (FLC) as listed in Tables 430.247 through 250 [430.6(A)(1)] and 430.22(A) through (G)].

Conductors that supply several motors must be sized to at least the sum of the four quantities enumerated in 430.24 (1) through (4). These are:
(1) 125 percent of the highest rated motor's full-load current (FLC) as listed in Tables 430.247 through 250.
(2) The sum of the full-load current (FLC) of the other motors as listed in Tables 430.247 through 250.
(3) 100% of the noncontinuous non-motor load.
(4) 125% of the continuous non-motor load.

The last sentence above each table allows us to use the ampacity columns for a range of system voltages without any adjustment. The conductor size must be selected from Table 310.16 per the terminal temperature rating (60°C or 75°C) of the equipment [110.14(C)(1)]. Don't use the motor nameplate full-load amperes (FLA) [430.6(A)(2)] to determine the motor conductor size.

Motor applications are considered continuous duty unless the nature of the control or apparatus the motor drives is designed so the motor will not operate continuously under load [Table 430.22(E) Note]. When a motor is not continuous duty because of this type of application, size the conductors using the percentages of Table 430.22(E). If a motor must stop when performing its function (such as in the case of an elevator motor) it is a good sign the motor is intermittent duty.

Conductors for a motor used in a short-time, intermittent, periodic, or varying duty application must have an ampacity of at least the percentage of the motor nameplate full-load ampere (FLA) rating shown in Table 430.22(E).

Overload
An overload is a condition where equipment operates above its current rating, or where the current exceeds the conductor ampacity. When an overload condition persists, equipment failure or a fire from damaging or dangerous overheating can result. A fault, such as a short circuit or ground fault, is not an overload [Article 100].

Overload devices protect motors, motor control equipment, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start, but not against short circuits or ground faults. Overload protection is not required where it might introduce additional or increased hazards, as in the case of fire pumps [430.31(A)].

Overload devices can be:

• Thermal overloads (heaters) in an overload relay of a motor contactor (starter). These heater units are selected using a chart or size given by the manufacturer.
• Solid-state (electronic) overloads have an adjustment dial for setting the trip level. They are installed in an overload relay of a motor contactor (starter).
• Inverse time circuit breaker and dual element fuses can serve as both motor overload protection and the motor short-circuit ground fault protection if the requirements of 430.32 are met [430.55].
• Fuses, when sized per 430.32(A) [430.36].
  

Overload sizing for continuous-duty motors
Motors rated more than 1 hp, used in a continuous-duty application without integral thermal protection, must have the overload device(s) sized per one of the four methods required in 430.32(A)(1) through (4)].

For example, you can use a separate overload device. This device must be selected to open at no more than 125% of the motor nameplate full-load current (FLA) rating depending on service factor or temperature rise:

• Service Factor. Motors with a marked service factor (SF) of 1.15 or more on the nameplate must have the overload device sized at not more than 125 percent of the motor nameplate current rating. Motor service factors are safety factors; they indicate how much the motor capacity can be exceeded for short periods without overheating. For example, a motor with a service factor of 1.15 can operate at 15 percent more than its rated output without overheating. This is important for motors where loads vary and may peak slightly above the rated torque.
• Temperature Rise. Motors with a nameplate temperature rise of 40°C or less must have the overload device sized no more than 125 percent of the motor nameplate current rating. A motor with a nameplate temperature rise of 40°C means the motor is designed to operate so it will not heat up more than 40°C above its rated ambient temperature when operated at its rated load and voltage. Studies have shown that when the operating temperature of a motor is increased 10°C above its rating, the motor winding insulating material's anticipated life is reduced by 50 percent.
  

Example
Question: A motor has a nameplate that specifies a service factor of 1.12 with a temperature rise of 41°C and a nameplate full load current rating of 25A. What size dual-element time-delay fuse is required for the overload protection of this motor?
(a) 20A (b) 25A (c) 30A (d) 40A
Solution:
Since the service factor of 1.12 is less than 1.15, and 41°C is over 40°C, the overload protection is sized based on 115 percent of the motor nameplate ampere rating [430.6(A)(2)].
Overload Protection = 25A × 115%
Overload Protection = 28.75A; use a 25A dual-element time-delay fuse [240.6(A)]
Answer: (b) 25A

Other Motors. No more than 115 percent of the motor nameplate current rating.

Branch-circuit short-circuit and ground-fault protection
A branch-circuit short-circuit and ground-fault protective device (OCPD) protects the motor, the motor control equipment, and the conductors against short circuits or ground faults, but not against overload [430.51].

We'll get into the details in Part 2 of this article.

Fundamentally different
It should be clear at this point that conductor protection for motors is fundamentally different from conductor protection in other applications. Why is this?
When voltage is first applied to the field winding of an induction motor, only the conductor resistance opposes the flow of current through the motor winding. Because the conductor resistance is so low (practically a dead short), the motor will momentarily have a large inrush current.

Once the rotor reaches its rated speed, the starting current reduces to running current due to counter-electromotive force (CEMF).

If the rotating part of the motor winding (armature) becomes jammed so it cannot rotate, no counter-electromotive force (CEMF) will be produced in the motor winding. This results in a decrease in conductor impedance to the point that it is effectively a short circuit. The motor then operates at locked-rotor current (often six times the full-load ampere rating) depending on the motor's Code letter rating [430.7(B)]. This condition will cause the motor winding to overheat and be destroyed if the current is not quickly reduced or removed.

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